This invention relates to the technical field of mobile radio communication. More specificly the invention relates to methods for efficient and flexible use of the frequency spectrum available for communication in a time division multiple access mobile radio communication system. The invention also relates to a base station and a mobile station for flexible and efficient use of the frequency spectrum available in a time division multiple access mobile radio communication system.
Many mobile radio systems are known and in use. The frequency band available for connections in a mobile radio communication system limits the capacity of mobile radio systems. Two base stations or mobile stations transmitting on the same radio channel of a FDMA system or on the same time slot of the same radio channel in a TDMA system may cause interference to each other. This kind of interference is sometimes called co-channel interference because the interference comes from the same radio channel. If the interfering mobiles or bases are sufficiently close in relation to radio propagation properties, the signal strength of the signals relating to one of the connections will not be sufficiently stronger than the interfering signals relating to the other connection. The information on the connection forwarded with the signals transmitted will then be difficult to understand. If the interfering mobiles or base stations are sufficiently distant from each other however, the signals relating to a connection will be sufficiently stronger than the interference signals of the other connection. The information of the connections will then be easily understood.
In order to use the same radio channel in FDMA systems, and the same time slot of a radio channel. In TDMA systems for more than one connection, some mobile radio systems are designed as cellular systems. The geographical area to be covered by a cellular system is divided into smaller areas called cells. Mobiles in a cell communicate with a base station for that cell. Some or all of the available radio channels are distributed among the cells according to a frequency plan.
Normally a conventional frequency plan means different radio channels are allotted to a cluster of adjacent or neighbour cells. No two cells in the same cluster may use the same radio channel. Each radio channel used by the base station or a mobile of one cell in a cluster, is different from every channel used by a base or mobile in another cell in the same cluster. However, cells in different clusters may use the same radio channels. Thus there may be simultaneous multiple use of a radio channel. Such multiple use is sometimes called channel re-use. The distance between cells using the same radio channel is sometimes called re-use distance.
Many different shapes and sizes of cell clusters are known to those skilled in the art, e.g. 3-cell, 4-cell, 7-cell, 9-cell, 12-cell and 21-cell clusters. Somewhat simplified the largest call handling capacity for a cellular TDMA system is achieved when using the smallest cluster which provides sufficiently low co-channel interference.
Athough the frequency plans described provide the important advantage of plural use of radio channels, often called frequency or channel re-use, such fixed frequency plans are cumbersome. Due to geographical variations, the cells, or zones covered by each base station antenna, will vary in size and shape. The coverage area of the system will thus normally be covered by several different known cluster combinations. Commonly, the cluster configuration, or decisions of which re-use patterns to be used, must be made with the aid of complex computer-analysis of the topography in the system.
Also other disadvantages are inherent in the use of fixed frequency plans. Normally, the number of desired connections in a cell varies with time. One cell may not be able to handle all desired connections because all channels and all time slots on TDMA channels alotted to the cell are occupied. At the same time the number of desired connections in an adjacent cell or a neighbour cell or any cell in the same cluster may be substantially less than the total capacity on all channels allotted to that cell according to the fixed frequency plan. Thus all desired connections can not be handled by the cell cluster in spite of the fact that there is at least one free channel or at least a free time slot on a radio channel which could have been used for the desired connections had this not been forbidden by the fixed frequency plan.
One way of reducing the above mentioned disadvantage of fixed frequency plans is not to distribute all radio channels available for connections in a mobile radio communication system, but to reserve a couple of radio channels. All channels but the reserved are distributed according to a frequency plan. The reserved radio channels may be temporarily used by any cell requiring more channels than the channels permanently allotted to that channel in accordance with the frequency plan. Such temporary use of a reserved channel does not cause co-channel interference for another cell already using that reserved radio temporarily channel. While this method of reserving and temporarily allotting some radio channels provides more flexibility with regard to variable connection handling capacity than a fixed frequency plan for all available radio channels, the total handling capacity for the whole system may decrease.
Another method of obtaining high traffic handling flexibility in a various areas of cellular mobile radio system is to completely abolish frequency planning and let all radio channels available for connections be a common resource to all cells. Any cell may use any radio channel available for connections provided there is sufficiently low co-channel interference from others using the same radio channel. This is sometimes called "dynamic channel allocation". While this method affords advantages with respect to changing call handling capacity for a cell, this method also has drawbacks.
Power conservation is an important aspect of small light weight portable battery powered mobile stations. In a normal telephone call, pauses in speech are frequent and quite long in relation to a radio channel time slot. Transmitting radio signals when there is no information to forward is a waste of battery power. Discontinuous transmission means the transmission is interrupted when there is a pause in the speech of a call or no information is being forwarded in ongoing connection.
Another way of saving battery power in a mobile station is to control the strength of transmitted radio signals in response to measured signal strength at the receiving base station. If the signal strength at the receiving base station is neglected, a mobile must always transmit radio signals with a strength sufficient for a worst case condition, e.g. when the mobile station is located at the fringe of a cell. For most locations such a signal strength is excessively high. If the strength of received signals are measured, a base station may send power control messages to the mobile permitting a reduction of the mobile transmit power whenever an excessive signal level is detected.
Some cellular mobile radio communication systems using digital modulation of radio signals transmitted are now in commercial wide scale use. One type of mobile radio communication system used in the USA is specified in the document EIA/TIA, Cellular System, Dual-Mode Mobile station--Base Station Compatibility Standard, IS-54, published by ELECTRONIC INDUSTRIES ASSOCIATION, Engineering Department, 2001 Eye Street, N.W. Washington, D.C. 20006, USA. This system has both FDMA radio channels for radio signals with analog modulation and TDMA radio channels for radio signals with digital modulation. For exhaustive information on this system, reference is given to the aforementioned publication, the subject matter of which is incorporated herein by reference.
The pan European digital cellular system abbreviated GSM is a type of digital mobile radio communication system in use in Europe. This system is specified in the document Recommendation GSM from ETSI/TC GSM, published by European Telecommunication Standardization Institute, ETSI B.P. 152-F-06561 Valbonne Cedex, France. For exhaustive information on this system, reference is given to the aforementioned publication, the subject matter of which is incorporated herein by reference.
Both the system according to TIA IS-54 and the GSM system are TDMA systems with many radio channels disposed in separate frequency bands. In a TDMA mobile radio system one obvious way of using the radio channels allotted to a cell would be to use one time slot of one radio channel allotted to the cell for a particular connection as long as possible, i.e. until termination or handoff of a connection. This is also done according to the aforementioned EIA/TIA IS-54 standard.
In a conventional TDMA system, where the same radio channel and time slot are used throughout a connection, any co-channel interference will last as long as both the connections last because the transmissions occur more or less simultaneously on the same radio channel. This means a worst case situation must be considered in frequency planning and cell cluster design. Frequency hopping has been suggested to circumvent this case.
According to one optional embodiment of the GSM system, time slots on plural radio channels alotted to a cell are used for one and the same connection. Any base and mobile transmits a sequence of radio signal bursts. Each burst is confined to a time slot, but the bursts are distributed on a plurality of radio channels. This affords advantages far as multipath propagation is concerned.
Embodiments of a GSM system with frequency hopping are discussed in the article "High performance cellular planning with Frequency Hopping", by Didier Verhulst and Colin Rudolph, published in Proceedings DMR IV, 26-28 Jun. 1990, Oslo, Norway, the subject matter of which is incorporated herein by reference. According to the article, frequency hopping provides advantages such as smaller optimum cluster size and more flexible frequency planning. The maximum connection handling capacity becomes interference limited and implementation of discontinuous transmission affords increased maximum capacity with even the smallest cluster sizes. The smallest cluster size investigated is 7-cell but the article also mentions that a "plain 3 cell cluster can in fact also be envisaged".
Another type of digital mobile radio communication system, somewhat different from the above described systems, using time division multiple access radio channels is the broadband code division multiple access system, abbreviated CDMA. In normal broadband CDMA systems, all the radio signal transmissions relating to different connections involving the mobile stations are not separated in time slots or in different narrow band radio channels. Also, in a normal broadband CDMA system there is no fixed frequency plan. Instead the base and mobile station, both in the same cell, and in surrounding cells deliberately transmit radio signals relating all connections simultaneously on the same wideband radio channel. As a consequence the co-channel interference in a CDMA system will be very high in relation to such interference in previously described TDMA systems. More precisely the interference level in CDMA systems will normally be several times higher than level of the desired radio signal relating to the connection.
The reason why a CDMA system can handle high levels of co-channel interference is the wide bandwidth of each radio channel used. The wideband radio channel in CDMA will normally have a bandwidth equivalent to several of the narrow bandwidth radio channels used in TDMA or FDMA systems. The wide bandwidth allows for a high degree of channel coding. Such coding makes it possible for the mobile and base station-receivers to recognize the desired signal from all other signals even though the interference level exceeds the level of the desired signal. A feature of the CDMA systems is that the number of connections permitted within a frequency band is not limited by the number of time slots/radio channels. Instead the call handling capacity is limited by the maximum level of co-channel interference which still permits the mobile and base station receivers to detect their desired signals.
In a CDMA system, power control and discontinuous transmission reduces the average total power of interfering signals. Thus, discontinuous transmission means reduced co-channel interference and increased capacity in a CDMA system, since the capacity generally depends on the average interference level. This is an advantage CDMA systems share with some frequency hopping TDMA systems in relation to prior art TDMA systems without frequency hopping.
Some different types of mobile radio systems similar to CDMA are discussed in the article "Slow Frequency Hopping Multiple Access for Digital Cellular Radiotelephone", by Didier Verhulst, Michel Mouly and Jacques Szpirglas, published in IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS VOL SAC-2, NO 4, JUL. 1984, pages 563-574, the subject matter of which is incorporated herein by reference. Various types of systems with somewhat different frequency hopping protocols, implementation of discontinuous transmission and power control are mentioned. One system protocol called "random SFHMA" does not have a reuse cluster and each user has his own personal sequence that is uncorrelated with the sequences of the other users. However, co-channel interference from mobiles in the same cell is not avoided. According to the slow frequency hopping scheme, abbreviated SFH, the mobile stations do not transmit separated radio signal bursts in time slots of frames on TDMA radio channels but transmit more continuously without burst separations of a length corresponding to time slots. The hopping pattern for transmission from mobile stations is part of the channel coding used to suppress the co-channel interference.
A slow frequency hopping scheme convertible for use in combination with TDMA is discussed in the article "Cellular Efficiency with Slow Frequency Hopping, Analysis of the Digital SFH 900 Mobile System" by Jean-Louis Dornstetter and Didier Verhulst, published in IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATYTIONS VOL SAC-5, NO 5, JUN. 1987, pages 835-848, the subject matter of which is incorporated herein by reference. When analyzing possible performance of this system, discontinuous transmission and power control are assumed to be implemented. In this article the minimum cluster size mentioned is the 3-cell cluster.
A method and apparatus for frequency hopping in a cellular radiotelephone system is disclosed in PCT patent application WO 91/13502. The object is to increase the number of available carriers to hop between in each coverage area. Instead of permanently allocating to each coverage area (cell) within a reuse diameter (cluster) a fraction of the carriers available within the reuse diameter, all or almost all of the carriers are allocated to each coverage area at non-coinciding time intervals. The hopping is performed in at least rough time-synchronism from sector to sector and from cluster to cluster to avoid same channel interference and adjacent channel interference within the reuse diameter. The method relies fundamentally upon sharing a carrier among various coverage areas synchronously in time, but does not require a slotted TDMA channel structure. Thus the cells within a cluster are allowed to share available frequencies while still maintaining a re-use pattern. The re-use pattern is maintained on a frame basis, i.e. the hopping may be viewed as using a new frequency plan for each frame. This method has the drawback of requiring synchronized base stations, in particular within each cell cluster, and to some extent between adjacent clusters. Another disadvantage is that no cell can simultaneously serve a number of mobiles corresponding to the total number of radio channels available to the cluster. Any call can only serve a number of mobiles corresponding to the number of radio channels simultaneously available to that cell, which is only a fraction of the total number.